Surface Mixed Layer Salinity Budget in the Tropical Indian Ocean

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The Madden-Julian Oscillations (MJO) is an atmospheric disturbance over the tropical oceans with a recurrence period between 30 and 90 days, consisting of surface westerly wind bursts, deep atmospheric convection, and heavy precipitation. The MJO generally originates in the Equatorial Indian Ocean, develops as it travels eastward at roughly 5 meters per second along the equator until reaching the central Pacific and then accelerates as it circumnavigates the globe, impacting global weather patterns. MJO prediction is unsatisfactory, especially at its initiation stage when the oceanic feedback to the atmosphere may be important. The DYNAMO (DYNAmics of the Madden-Julian Oscillation) experiment conducted in 2011-2012 in the central Tropical Indian Ocean was successful in capturing several initiations of the MJO. In that context, one of the goals of DYNAMO was to understand the effects of salinity stratification on the turbulent entrainment flux and Sea Surface Temperature in the Tropical Indian Ocean where salinity stratification plays a significant role in the heat, momentum and salinity budgets of the surface ocean mixed layer. Observations from the DYNAMO experiment confirm that the turbulent heat flux at the base of the surface mixed layer is a primary component in the heat budget. The turbulent heat flux, however, varies by amounts larger than 100 Watts per square meter at intra-seasonal (e.g., MJO) time scales under similar surface wind and buoyancy forcing, and is presumably modulated by the barrier layer processes. This study will evaluate the details of the surface mixed layer salinity budget and barrier layer properties, and their temporal and spatial variations. Results from this proposed study will quantify and increase understanding of salinity stratification effects on turbulent entrainment fluxes in order to improve parameterizations of turbulent fluxes in the central and eastern Tropical Indian Ocean. This analysis project will complete the training of a Ph.D. student.This project focuses on the surface mixed layer salinity budget in the central and eastern Tropical Indian Ocean and its modulation of Sea Surface Temperature and the evolution of the Madden-Julian oscillation. Comprehensive in-situ observations from ocean moorings deployed in the central and eastern Tropical Indian Ocean as part of the DYNAMO experiment, and remotely-sensed Sea Surface Salinity from the Aquarius satellite will be used for that purpose. The moorings collected meteorological variables, as well as temperature, salinity, and horizontal velocity measurements in the upper ocean. All the DYNAMO moorings and some RAMA moorings were equipped with microstructure sensors, allowing direct measurements of turbulent entrainment fluxes. These observations provide a unique opportunity to close the surface mixed layer salinity budget in the central and eastern TIO during different MJO phases in the period of the DYNAMO experiment. The air-sea salinity flux will be computed using mooring meteorological measurements. The advective salinity flux will be computed using mooring measurements, aided by the Aquarius measurements. The turbulent salinity flux will be computed using direct microstructure measurements. The dominant fluxes in the surface mixed layer salinity budget will be identified. The temporal (from days to intra-seasonal) and spatial (central to eastern Tropical Indian Ocean) variability of salinity fluxes and barrier layer properties, and their effects on shear instability and turbulent entrainment flux will be quantified in different MJO phases. This study will establish the basis for developing a physics based turbulence parameterization scheme that includes the effects of barrier layer process that can help improve model predictions of the initiation and evolution of the MJO.